I'm guessing here that when you say "Book Titles", you mean Books that have been created in the Table of Contents structure?
If so, I have never seen anything work that way. At least not with RoboHelp.
I'm not sure I can even fathom a way to make that work. Typically, the search results only contain Topic Titles. As a Topic reference may reside in many different TOC books, I'm wondering how that would even be sorted (assuming it were an option).
Sorry, but I think the answer here (unless someone else pops in with a workaround) is that it can't be done.
I am even wondering why you'd want to. The only way I can think of doing what you want is to have each book with a single topic with the title the same as the book! As you will nearly always have two or more topics in a book, this is a non-starter. As Rick says, the search is always topic based so I can't see how this would work.